Comprehensive Review Of Small Field Dosimetry...European Journal of Molecular & Clinical Medicine...

European Journal of Molecular & Clinical Medicine ISSN 2515-8260 Volume 07, Issue 07, 2020

3595

Comprehensive Review Of Small Field

Dosimetry

Bhagat Chand1,2, Mukesh Kumar1, Muninder Kumar2, Priyamvda2

1Department of Physics, School of Chemical Engineering and Physical Sciences

Lovely Professional University, Punjab-144411. 2Department of Radiotherapy, Dr. Rajendra Prasad Government Medical College (Tanda),

District Kangra, Himachal Pradesh-176002.

Corresponding address: M Kumar ([email protected]),

B Chand ([email protected])

Abstract: The small static and composite fields occur in clinic while implementing

Intensity Modulated Radiotherapy (IMRT), Volumetric Modulated Arc Therapy (VMAT),

Stereotactic Radiosurgery (SRS) and Stereotactic Body radiotherapy (SBRT). These small

fields are limited by constrains related to radiation beam origin and the measurement

instruments available in the field. The clinical need of these fields is in achieving the

homogenous and conformal dose in the tumour. The radiation beam source cannot be

modified in any radiation unit until a change in its design is done. So, the range of

radiation detectors like ionization chambers, scintillation detectors, silicon diodes and films

have been extensively used for small field dosimetry. The plastic scintillator Exradin W2

has been tested to be the best available detector among the range of detectors for small and

very small fields. The use of any detector in small field dosimetry must be accompanied by

another standard detector and the data should be supported by Monte Carlo simulation

studies if possible. Also, appropriate choice of Patient Specific Quality Assurance (PSQA)

device is necessary to ensure the accurate dose delivery to the patients.

Keywords: Small field dosimetry, Detectors, Plastic Scintillators, patient specific QA.

Introduction

The use of small static and composite fields has significantly increased in past few years. The

inception of modern sophisticated radiotherapy units like GammaKnife (Elekta AB

Stockholm, Sweden), CyberKnife (Accuray Inc. CA), Tomotherapy (Accuray Inc. CA) and

Brainlab (Brainlab AG, Germany) and sophisticated collimator systems have made it very

easy and user friendly to implement the Stereotactic treatments in clinic. Although, the

complexity in technical operations, these systems promises very conformal dose delivery to

the tumours while the resulting sharp dose fall-off helps sparing surrounding normal

tissues[1-4]. The tailored dose distribution can be achieved very efficiently by the small field

openings ranging from 2 cm to submillimeter levels.

The static collimator openings in modern GammaKnife units ranges from 4 mm to 16 mm in

a hemispherical arrangement, in TomotTherapy the variable openings can be achieved by

binary operation of multileaf collimators (MLCs). The CyberKnife units have iris of different

openings and can deliver radiation from various directions. The Brainlab microMLCs have

variable openings with a minimum leaf width of 2.5 mm. The planar and non-coplanar beam

delivery results in more conformity and reduced plan complexity [5-8]. The conventional

linear accelerators have the standard MLCs of 1 cm, 0.5 cm and 0.25 cm width to modulate

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the radiation beam fluence. The fluence is delivered by a series of static or dynamic leaf

segments forming a composite field.

The clinical advantages of these technologies are only possible if the small radiation fields

are modelled accurately in the treatment planning systems (TPS) and their dosimetry is

performed with profound accuracy and confidence with high precision. The high dose rate

flattening filter free (FFF) beams, which are new to the conventional radiation units are also

being extensively used in radiosurgery application, owing to their advantage of high dose rate

and hence reduced treatment time [9-10]. Also, there is change in beam spectra with

decreasing field size which poses another challenge. The small field dosimetry is limited by

various constraints related to radiation beam source and the dosimeters used for

measurements. It is emphasized that the acquisition of radiation beam data should be done by

a Medical Physics expert, rather than simply accepting the vendor’s data for patient

treatment, especially for treatments involving small and complex fields [11 , 12].

Numerous researches have been published describing the dosimetry of small fields, detectors

for small field dosimetry and its clinical implementation. This article aims to review the basic

physics of small photon fields, their dosimetry formalisms and approaches for PSQA in

treatments involving small fields, keeping in mind the current commercial availability of the

equipment.

Physics of small fields Theory of Small Radiation Fields: There is no consensus definition as to what constitute a

small field. However, the field size less than 3 × 3 cm2 is widely being used and is considered

outside conventional treatment field and needs special attention [13-15]. In more details, the

Institute of Physics and Engineering in Medicine (IPEM) defines a small field as “the field

having dimensions smaller than the lateral range of charged particles that contribute to the

dose deposition at a point of measurement along the central axis”. Generally, a field size

smaller than 4×4 cm2 is treated as nonconventional. This field size less than 3 × 3 cm2 for a

6MV photon beam are referred to as small field [16-17,19].

Charles et al. [14] has defined a field size of lateral dimensions smaller than 15 mm as very

small field. He has simulated a 6 MV beam in BeamNRC [18] MC code to include the effects

of collimator scatter and source occlusion individually. This approach demands special

attention in selection of detectors for very small fields and also, to check the accuracy of

beam models for such small beam openings.

The small fields are limited by photon source related and detector related constraints. The

small field parameters are affected by partial occlusion of radiation beam source, which cause

a decrease in radiation output and extension of penumbra width over field edges and a

breakdown of the classical definition of radiation field size by Full Width Half Maxima

(FWHM) [13, 15-17, 19-20]. Also, beam hardening has been reported with decreasing

collimator openings. Another limiting factor is the availability of suitable detectors for small

field dosimetry. Most active dosimeters have limited resolution in small and relatively

inhomogeneous beam and cause a measurement error along the measurement direction.

Under the effects of volume averaging and response variations, low density detectors under

respond and high density detectors over respond in the medium [21-22]. The effect is more

pronounced in heterogeneous media. Alongside, the density effect of detectors produces large

perturbations breaking down the ideal Bragg-Gray conditions [23-25].

These conditions pose a serious challenge to the Medical Physicists in selection of

appropriate radiation detectors, strategies for beam data measurements in small and very

small static and composite fields. Concerns raise regarding the accuracy of beam modelling,


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specially the transport of secondary charge particles generated by interactions in TPS

according to the requirements of the algorithms.

Formalism for Small Field Dosimetry

International Atomic Energy Agency (IAEA) and American Association of Physicists in

Medicine (AAPM) has published a report about the dosimetry of small and non-standard

fields [28]. This report gives the guidelines for small field dosimetry in conventional and

dedicated radiosurgery systems. The formalism follows the recommendations of the

international dosimetry code of practice (CoP) of IAEA [26] and AAPM [27] based on

absorbed dose to water (𝑁𝐷𝑤𝑄0) calibration.

This formalism has suggested three different field definitions to be considered for dosimetry

of specialized and complex treatments. The machine specific reference field (fmsr) has been

suggested for those fields, where the conventional reference field (fref) of 10 × 10 cm2 cannot

be established. This field must be as closely possible equivalent to fref. Also, a traceability

chain has been given between the fmsr and the calibration laboratory. The concept of plan class

specific reference (fpcsr) and clinical field (fclin) have also been given. The fpcsr can be either be

a single field or segment sequence for which full charged particle equilibrium can be

achieved in dosimetry. The fpcsr can be a 3D irradiated volume or a 4D delivery sequence

formed by small composite fields. The fclin tends to be the clinical field at the time of dose

delivery to the patient.

This new formalism is most applicable for the nonconventional delivery systems like

GammaKnife, CyberKnife, BrainLab and TomoTherapy. The formalism has same

recommendations as that of international CoP for conventional linear accelerators performing

SRS.

More recently, IAEA has adopted and modified the recommendations of Alfonso et al. [13] in

a new CoP for reference and relative dosimetry of small radiation fields used for external

beam radiotherapy with energies of nominal potential of 10 MV [28]. This approach has also

considered the FFF beams. This protocol does not have mention of any other modalities like

electrons and protons etc. It is based on the use of a ionization chamber that has been

calibrated in terms of absorbed dose to water NDw,Qo or NDw,Qmsr in a standard’s laboratory’s

reference beam of quality Qo or Qmsr respectively. It also provides guidance for

measurements of field output factors and lateral beam profiles at the measurement depth. The

aim is to establish uniform approach of small field dosimetry.

2.3. Detectors for small Field Dosimetry

The stereotactic fields demand for a detector with very high resolution and very small volume

having high sensitivity [20, 29]. Many researchers have studied the parameters like excellent

spatial resolution, a linear response with dose and dose rate, tissue equivalence, stable short-

term readings, isotropic response, low energy dependence and minimal background signal. A

wide range of detectors have been used in small fields including small volume ionization

chambers, solid state detectors like shielded and unshielded diodes, scintillators and

diamonds, MOSFETs, TLDs, OSLDs, radiochromic films and chemical dosimeters [28,30].

Some recent studies have discussed the feasibility of Dose Area Product and Cherenkov

radiations in small field dosimetry owing to its advantages of nearly zero perturbations in the

radiation field.

Ionization Chambers: The ionization chambers have been in use for radiation dosimetry

from a long time and are current standard for dosimetry of radiation beams. The calibrations

are all traceable to the ionization chambers of standard labs. These have been found to be

most reliable for dosimetry of small fields. Also, the calibration in terms of absorbed dose to

water NDW is most commonly available for ionization chambers only. Moreover, the ion

chambers are easily commercially available, less expensive, known history of stable response


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in a range of radiation beams, user friendly and hence has always be used either a primary or

reference dosimeter in small field dosimetry [31-34].

The traditional thimble type ion chambers of volume 0.3 cc – 0.6 cc are not suited for small

field dosimetry, due to large perturbations and volume averaging that leads to

underestimation of radiation dose at measurement point along beam central axis (CAX). The

small volume mini and pin-point ionization chambers have been tested for their response and

shows stable response for field size down upto 2 × 2 cm2, but below that these also shows

perturbations and volume averaging. However, the micro chambers of volume 0.002 cc –

0.01 cc to some extent shows promising results in small and very small fields but are limited

by their low sensitivity and effects of stem and wire irradiations[34-39].

Liquid ion chambers have been tested in small fields owing to their nearly water equivalent

density and hence unit mass stopping power and mass energy absorption coefficient ratios.

These detectors have high resolution even at small volumes and not compromised by

sensitivity simultaneously. But the liquid ion chambers are currently unavailable

commercially and hence cannot be evaluated further [40-41].

Silicon Diodes: The semiconductor technology has revolutionized the world with its compact

designs and rapid response [21, 32, 42-45]. These have been used in several applications in

radiotherapy. The diode detectors have been used for relative and in vivo dosimetry very

extensively. Commercial diode detectors are available with various vendors in range of

volumes ranging from 0.02 mm3 and much smaller. The diodes are necessarily very small

volume, highly sensitive and hence suited for small field dosimetry. The diodes are seen to

show higher directional dependence and hence are advised to use in an orientation with its

symmetric axis parallel to beam central axis.

The unshielded diodes (electron diodes) have been seen to show relatively better response in

small fields than the shielded diodes (photon diodes). The photon diodes under respond the

low energy scattered radiations due to tungsten shield [44].

Diamond Detectors: The diamond detectors have small volume and uniform structure that

makes it suitable for dosimetry with minimal perturbations [47-48]. The diamond detectors

have electron density nearly equivalent to that of human tissue and hence the corrections

required are very small, also, due to this, the mass stopping power and mass energy

absorption ratios varies to very small extent making it nearly energy independent. These

characters make diamonds very suitable for small field dosimetry. The natural diamonds,

which were initially used, has to be provided a bias voltage of 800 V, but the modern

diamond is synthesized by chemical vapour deposition (CVD) technique and shows best

results at zero bias [49-50].

Plastic Scintillator Detectors: Plastic and organic scintillators are based on the principle of

scintillation dosimetry, where the detected photon energy is transported through a

photomultiplier tube to produce the signal proportional to the absorbed dose. These

scintillators have seen to show very stable response in small fields. These have small yet

highly sensitive volume and profound stability making it superior to other detectors [44, 51-

54]. The plastic scintillators require very small corrections due to their nearly tissue

equivalent density and has seen matching with the Monte Carlo simulation results [25, 36].

The only issue in scintillators is the presence of Cherenkov light radiations, the methods for

correction of which have been proposed [48].

Chemical Dosimeters: Various chemical dosimeters including the radiochromic films have

been tested in small fields. The EBT 2 and EBT 3 radio chromic films have shown very

stable response and provide indeed the highest resolution [44, 48, 55-57]. However, these

films are limited by their mild response to Ultra-Violet (UV) radiations which may produce

errors in photon dose estimation. Also, the films are limited by their dose and dose rate

ranges.


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Fricke dosimeter and its derivatives have been tested in small field dosimetry and has shown

very good response. The response of chemical dosimeters is read by the UV spectrometry

method, which has seen limiting the dose range and resolution of these chemicals [58-59].

The suggestions of using Fourier Transformation Infra-Red (FTIR) or Nuclear Magnetic

Resonance (NMR) for readout is however to enhance the accuracy of these detectors but

simultaneously imposes a burden on the department budget and space requirements [58, 60].

Equipment for PSQA

The delivery of high dose per fraction in small fields is challenging the dosimetry in every

aspect. In previous sections we have discussed the physics and dosimetry of small fields for

commissioning and reference dosimetry. It is highly recommended by several national and

international bodies to perform the pretreatment PSQA to check the accuracy of dose

calculation, plan transfer and treatment delivery. The PSQA is necessarily required to ensure

the patient safety and enhance treatment quality [61-62].

Various commercial systems are available for PSQA, ranging from the point ion chambers to

diodes, two-dimensional films and detector arrays and three dimensional dose reconstruction

methods [40, 49, 52]. These commercial systems are either air or liquid filled ionization

chamber based or based on diode detectors in a two-dimensional fashion. PTW (PTW

Dosimetry, Germeny), IBA (IBA Dosimetry, Germeny), Sun Nuclear (SNC, USA) and

Standard Imaging (Standard Imaging Inc., USA) are the key commercial vendors that provide

2D and 3D detector arrays for pretreatment dose verification. Most vendors are providing

solutions for small field dose verification either in the same conventional detector array or as

a separate detector array. The associated, data analysis software utilize the approach of dose

difference and distance to agreement or combination of both in a single parameter called

gamma index (γ) analysis. 2D and 3D gamma analysis and anatomical dose maps are

provided in most of the systems [63-64].

The major requirement of a PSQA systems is to have a dosimetry system with highest

resolution, rapid response, real time data analysis and fast setup. The conventional film

scanner based system however provides the best resolution but have limitations in other

parameters like its response and readout times which cannot give real time results. The

commercial systems fulfil the requirements with an accuracy equivalent to or better than the

conventional film scanner system [65].

AAPM TG 218 [65] has extensively discussed various guidelines for the PSQA in IMRT on

various systems and also compared the vendors providing the PSQA dosimeter systems.

These guidelines provide a set of tolerance and action levels for IMRT plan verification.

These guidelines should be considered while establishing the PSQA plan in clinic for best

outcome of results.

Discussions

The small fields which usually occurs in the SRS treatments and in standard IMRT forms the

composite fields. The main constraints on these fields are source occlusion and availability of

suitable detectors. These constraints have been identified very early by Duggan [20] and Das

[29]. The impact of these parameters on the small field dosimetry is a reduction in beam

output, breakdown of FWHM to define field size by extension of penumbra edges and beam

hardening. The non-availability of sophisticated detectors has made it more difficult to

perform reference and relative dosimetry [13, 17, 19, 66]. With increasing use of small fields

in clinic a unified formalism for small field dosimetry was postulated by Alfonso et al. [13]

that discussed the use of fmsr in clinic and its traceability to the calibration conditions.

The definition of very small field size by Charles et al. [14] seems to be an awakening work

to attract attention towards these fields, this definition however needs a more specific

approach considering all types of beam defining arrangements such as MLCs, Jaws and

Cones etc. Also, the knowledge of all suitable detectors available, to give more accurate


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definition of small fields. It should consider the factors that determines the scale that a

radiation field has to be considered small or not.

The IAEA AAPM TRS 483 has modified the formalism of Alfonso et. al. [13] with little

modifications and extension to FFF beams [28]. This CoP has been evaluated by Haq et al.

[67] and they found that it has good correlation with the conventional CoP of IAEA and

AAPM for reference and relative dosimetry. They provided correction factors for several

detectors in fmsr on two different linear accelerators and suggested their use in clinical

practice. International Commission on Radiation Units and Measurements (ICRU 2014) has

suggested a more regressive use of Monte Carlo simulations in small field dosimetry to

determine accurately the correction factors mentioned in these protocols [68]. This CoP has

provided a detailed formalism for dosimetry and choice of dosimetry system and should be

followed for small field dosimetry.

Regarding radiation source related constraint of small field dosimetry, the scope in changing

the design of source or beam collimation mechanism is very small in conventional settings.

The major work has to be done on the radiation detectors. Various investigators have studied

a set of commercial dosimeters for small field dosimetry [13, 16, 20, 21, 32, 34, 43, 69-70].

Emphasis is given on the use of very small volume, highly sensitive detector having stable

response for dosimetry of these fields. The detector must also not be affected by change in

beam spectra, as in small fields the beam hardening takes place.

Ionisation chambers have been standard choice for dosimetry since very long. These are

found to be very stable in all beam conditions. The micro ionisation chambers have been

tested in small fields and has shown good results in small fields down upto 2 cm. Bouchard et

al. [21] has studies the micro ionisation chambers down upto 1 × 1 cm2 in a daisy chain

method to determine machine specific correction factor and has shown that PTW 31014,

PTW 31006 and IBA CC01 ionisation chambers have excellent results in terms of

reproducibility and stability. But, volume averaging and reduced resolution tend to violate the

cavity theory conditons and hence cause an underestimation of radiation doses. Owing to this,

Charles et al. [14] has suggested to measure the beam profiles and output factors

simultaneously for any particular field size to reduce such variations. It is also suggested to

make use of atleast two detectors to measure radiation beam parameters in small and very

small fields. Moreover, the dielectric liquid filed ion chambers had been a classic solution for

small field dosimetry, but it is no longer commercially available.

In recent times the detector technology has been revolutionised with commercial vendors

providing alternatives to the conventional ionisation chambers for relative dosimetry in small

fields. The shielded and unshielded diodes have been tested very extensively in small fields

and have some superiority in terms of its resolution. PTW diode 60018 and IBA SFD

(unshielded diode) has shown good response in small fields upto 1 × 1 cm2, however the PFD

(shielded diode) has larger variations in small fields [56]. These solid state detectors are

limited for relative measurements of beam profiles and PDD data. In output factor

measurements there variations have been reported even with the latest available diodes [51 ,

71]. Due to large variations in their constructions and their limited lifetime, they are not

stable and hence can’t be used for reference dosimetry.

The PSDs have been tested in recent times very extensively [54 ,72]. PSDs have small

volumes, nearly tissue equivalent and has high sensitivity. Debnath et al. [48] has recently

developed an inorganic scintillation detector for use in small fields. They have tested it in

small fields upto 0.5 0.5 cm2 for output factors, absolute dose and PDD measurements and

found small variations of upto 0.5%. Casar et al. [60] have determined correction factors for

various diode detetors in accordance with IAEA AAPM TRS 483 guidelines. They have

adopted Exradin W1 PSD as standard dosimeter along with the EBT 3 films. They have

recommended their data as reference class and addenum to TRS 483.


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Recently Galavis et al. [72] have compared the Exradin W1 scintillator with novel Exradin

W2 scintillator. The issues of cherenkov radiations in W1 scintillator caused variations in

signal which has been temperature dependent. The Exradin W2 scintillator is provided with

an electrometer system to correct for the cherenkov radiations. Moreover its correction factor

has been found 1.0 and hence an ideal detector for measurement of output factors, beam

profiles and dept dose data [72].

Several investigators have studied different detectors for small field dosimetry and the only

thing common among all thse studies was the EBT films (EBT 2 and EBT 3)used as

reference dosimeter [56 , 57]. The films have quality of highest resolution along with its

stable response and once calibrated the whole batch can be characterised with the same

calibration curve. Hence it is suggestive to use the gafchromic films along with any other

detector for verification of small field parameters.

Concept of using dose area product (DAP) for small field dosimetry has been discussed

previously [73] In the scinario where most of the conventional dosimeters fails, the dose can

be estimated by using a large diameter plane parallel chamber the dose can be integrated as

function of area i.e. field size. It is also suggested to use ratio of DAP at 20 cm dept to that at

10 cm dept (DAPR20,10) for beam quality specifications in narrow beams [74]. This method

has been tested by against Monte Carlo simulations and found within 1% variation to the

conventional beam quality specifier; Tissue Phantom Ratio (TPR20,10).

The DAP is at present being used for dosimetry of interventional radiology applications, and

its use in Radiotherapy has never been reported earlier. Also, no specific detectors are

available for this application. However, the DAP concept is very unique at this time, which

can be used with minimal perturbation of the radiation beam. This concept must be taken for

further research and standardisation for small field dosimetry in clinic, the commecrial large

area plane parallel chambers designed for proton therapy dosimetry can be studied for this

application in photn beams.

Another emerging concept of estimating Cherenkov light radiation [75] for small field

dosimetry has been postulated. Glasar et al. [77] A water tank doped with tracer amounts of

fluorophore for better light emission has been utilized. The detection was done through lateral

detection using simple CMOS camera with room lights off. The authors have presented

different imaging approaches namely 1) simple detection of depth dose and off axis beam

profile and its agreement with reference; 2) the three dimensional tomography of such beams

using the filtered back projection method to recover 2D and 3D volumetric data by rotation of

phantom camera assembly and to obtain a real time two-dimensional imaging of IMRT and

VMAT plans in water phantom by Cherenkov imaging.

These methods show good agreement with the TPS values, but the 3D volumetric acquisition

and 2D image seems a less feasible method as it can only be done for a research dedicated

setup. This is a revolutionary concept as the Cherenkov radiations do not have any field size

or depth dependent response, thus can be utilized in small field dosimetry very efficiently and

accurately [76, 78, 79].

The pretreatment patient dose verification QA has been a paramount requirement for each

IMRT treatment as highlighted by AAPM [65]. The small treatments involving small fields

require special attention in this case, especially when the dose per fraction is very high.

Investigators [40, 80, 81] have characterized the commercially available detector arrays PTW

1000 SRS for SRS QA and has found it highly stable and suitable for such QA requirements.

They suggested that its spatial resolution is adequate enough to validate the static and

composite field QA. The results of this device were found to be equivalent to the EBT 2 film.

The γ passing criteria has also been discussed and it is emphasized that considering the large

fraction size and small number of fractions in stereotactic Ablative Body radiotherapy

(SABR), the demand for accuracy in gamma passing and its stringent limits has been


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increased and need careful implementations of the QA Programme as it involves the small

composite fields, dosimetry of which is limited by several constraints. Also, it has been

shown that the passing rates are dependent on the modulation index, and as the modulation

increases, the number of segments with small size increases. Thus, careful choice of detectors

and institute specific protocols are needed for such sophisticated system, considering the

problems in small field dosimetry. The authors guide towards a stringent criterion of gamma

passing in SABR and implementation of a 2D detector array [55, 82]. Also, it is seen that

SABR VMAT plans with a gamma passing criterion of 2%/1mm global, and find that this

criterion is most efficient and sensitive towards errors.

The implementation of exact standard approach of gamma analysis in the commercial

software is always a concern as AAPM 218 [65] has discussed. Also, more stringent

requirements of gamma analysis as discussed above are required to be implemented to

identify the errors which may reduce the pass percentage of analysed points. Schmitt et al.,

[83] has recently given guidelines for implementation of SRS and SBRT in clinic, these

guidelines must be followed for a comprehensive patient treatment course. The recent

standard guidelines [3, 5, 84] gave a thorough reference for all types of clinical and

Dosimetric aspects of small fields and should be implemented.

Conclusion

The small static and composite fields are commonly occurring in modern day radiotherapy.

Rather it be dynamic IMRT, VMAT or stereotactic treatments, the small fields play its role in

achieving dose conformity and reduce the normal tissue toxicity. It must always be borne in

mind that these complex treatments need due care and standard mechanism for their

implementation in clinic. The detectors must be chosen owing to clinical need, extensive

survey of literature and budget load of the institution. The plastic scintillators have been

proved to be the best choice till time for small fields, however, it must be cross examine with

other standard dosimeter like film of ionization chamber. The Monte Carlo simulation studies

are also an important aspect in radiotherapy dosimetry, hence its suggested to make use of

these simulations to validate the clinical data and determine correction factors. For PSQA, the

detectors available must be characterized and tested in small fields and clinic specific action

levels and passing criteria must be set following the standard guidelines. It is highly

suggested to follow standard guidelines while implementing the small fields in clinic.

Conflict of Interest No conflict of interest

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